Viruses
Jessica Harwood
Douglas Wilkin, Ph.D.
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AUTHORS
Jessica Harwood
Douglas Wilkin, Ph.D.
EDITOR
Douglas Wilkin, Ph.D.
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Printed: January 25, 2015
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Niamh Gray-Wilson
Jean Brainard, Ph.D.
Sarah Johnson
Jane Willan
Corliss Karasov
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C HAPTER
Chapter 1. Viruses
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Viruses
• Explain why viruses are not considered living.
• Describe the features and list examples of viruses.
What causes the common cold?
That miserable cough and runny nose is caused by one villain: a virus. Viruses come in many different shapes,
including the prickly balls you see here. They are so tiny that they can only be seen with a very powerful microscope.
What is a Virus?
We have all heard of viruses. The flu, the common cold, and many other diseases are caused by viruses. But what is
a virus? Do you think viruses are living? Which domain do they belong to? Bacteria? Archaea? Eukarya?
Are Viruses Alive?
The answer is actually “no.” A virus is essentially DNA or RNA surrounded by a coat of protein ( Figure 1.1). It is
not made of a cell, and cannot maintain a stable internal environment ( homeostasis). Recall that a cell is the basic
unit of living organisms. So if a virus is not made of at least one cell, can it be living? Viruses also cannot reproduce
on their own—they need to infect a host cell to reproduce. So a virus is very different from any of the organisms
that fall into the three domains of life.
Though viruses are not considered living, they share two important traits with living organisms. They have genetic
material like all cells do, and they can evolve. As the process of evolution has resulted in all life on the planet today,
the classification of viruses has been controversial. It calls into question the very definition of life.
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FIGURE 1.1
These little "alien" looking creatures are
viruses, and these specific viruses infect
Escherichia coli bacteria.
Shown is a
representation of viruses infecting a cell.
The virus lands on the outside of the cell
and injects its genetic material into the
cell.
Replication
Viruses infect a variety of organisms, including plants, animals, and bacteria. Once inside the host cell, they use the
cell’s own ATP (energy), ribosomes, enzymes, and other cellular parts to make copies of themselves. The host cell
makes a copy of the viral DNA and produces viral proteins. These are then packaged into new viruses. So viruses
cannot replicate or reproduce on their own; they rely on a host cell to make additional viruses.
Viruses and Human Disease
Viruses cause many human diseases. In addition to the flu and the common cold, viruses cause rabies, diarrheal
diseases, AIDS, cold sores, and many other diseases ( Figure 1.2). Viral diseases range from mild to fatal.
FIGURE 1.2
Cold sores are caused by a herpes virus.
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Chapter 1. Viruses
Summary
• A virus is composed of DNA or RNA surrounded by a coat of protein.
• Viruses are not considered living things because they cannot reproduce on their own, and they are not comprised of cells.
Explore More
Use the resources below to answer the questions that follow.
Explore More I
• Viruses at http://www.youtube.com/watch?v=L8oHs7G_syI (8:06)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57468
1.
2.
3.
4.
5.
How do viruses reproduce? How does this differ from other organisms?
What kinds of nucleic acids can viruses have?
Explain one of the theories as to how viruses came to be.
What is the importance of the "envelope" to a virus? What is the envelope made of?
What is a difference between the lytic cycle of a virus and the lysogenic cycle?
Explore More II
• How Flu Viruses Attack at http://www.youtube.com/watch?v=TVLo2CtB3GA (3:48)
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/57469
1. What is one way a flu virus can kill a human?
2. Do mutations make viruses more deadly? Why or why not?
Review
1. Is a virus a living thing? Why or why not?
2. Name four examples of human diseases caused by a virus.
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References
1. Image copyright Monika Wisniewska, 2014. Drawing of viruses attacking a cell . Used under license from
Shutterstock.com
2. Metju12. Cold sores are caused by a herpes virus . Public Domain
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